1. Field of the Invention
This invention pertains to the field of pipe joints or couplings. More particularly, this invention pertains to a flange coupling for diverse metal elements.
2. Description of the Prior Art
A process requiring an ultrahigh vacuum (UHV), a vacuum in the order of less than 10.sup.-9 Torr or approximately 10.sup.-12 atmosphere, is typically performed in a chamber having a stainless steel wall with the object being processed mounted on a "finger" or inward projection from the wall. Typically, the temperature of the object must be precisely controlled and measured, so that it is highly desirable to mount the object on an element of oxygen-free-high-conductivity (OFHC) copper which has high thermal conductivity and does not affect the UHV process. It is also desirable that a substantial portion of the OFHC copper element be directly accessible from the chamber exterior for direct connection of temperature maintaining and measurement materials and devices to the element from the chamber exterior.
Prior art arrangements for connections through a UHV chamber wall and for cold or hot fingers in lower vacuum processes do not provide a solution to the apparently simple need to connect an exteriorly accessible copper finger to a stainless steel UHV chamber wall for several reasons. First, copper cannot be welded to stainless steel. Second, brazing cannot be used since the brazing material leaks and out gasses excessively for UHV conditions. For similar reasons, gaskets, sealing greases, and the like effective at a lower vacuum are not practical at UHV conditions. Third, the finger must be readily and economically removeable and replaceable in the event it cannot be reused, becomes contaminated, or requires reconfiguration for a different process or object. Fourth, the presence of a leakproof layer, even of metal, between the finger and the chamber exterior results in an undesirable decrease in thermal conductivity therebetween.
A well known UHV flange connector for a pair of stainless steel elements uses a soft copper ring oppositely engaged by annular knife edges individually borne by the stainless steel elements, these elements having flanges adjacent to the knife edges and engaged by stainless steel rings through which extend bolts for drawing the knife edges into UHV sealing relation with the copper ring. This UHV connector is advantageous because the copper ring may be reused by machining its opposite faces to remove the indentations caused by the knife edges and because the stainless steel elements may be welded to a stainless steel UHV chamber wall. However, this connector does not provide a solution to the problem of connecting a dissimilar metal finger to such a stainless steel wall since the connected knife edged elements are themselves stainless steel and since substitution of OHFC copper, which is relatively soft, would result in a knife edge too soft for use with the copper ring.